Vertical expansion of landfills, also known as piggyback landfills, describe the building of another landfill unit above an existing one. The purpose of such expansions is to maximize the waste that can be safely stored over the same permitted landfill footprint. While vertical expansions offer many advantages, it is important to implement this technique on well-maintained existing sites where fundamental practices such as “maximizing compaction efforts and controlling the materials coming into a facility” have been observed (Law, Goudreau, Fawole, & Trivedi, 2008). This is crucial due to the new loads and stresses that the added mass can impose on the system and the subsequent implications on differential settlements, “slope stability, foundation support, waste settlement, integrity of the bottom liner and leachate header pipe systems, and stormwater management” (Law, Goudreau, Fawole, & Trivedi, 2008). The vertical expansion of landfills can be accomplished through various techniques, but is dependent on the site conditions and their relationship to the previously listed design considerations that must be accounted for. Some common techniques aimed at maximizing airspace include implementing perimeter retaining walls and mechanically stabilized earth (MSE) perimeter berms.
Implementing perimeter retaining walls as a method of maximizing airspace at a landfill often uses conventional soil stabilization methods with concrete blocks. Because this method avoids any horizontal expansion of the landfill, the permit required for the modification is easier to obtain than one for a modification that would include such an expansion. The vertical stacking of masonry blocks is the main characteristic feature that distinguishes perimeter walls from MSE walls.
The basic design of an MSE berm is a soil slope composed of compacted soil that is internally reinforced in order for the system to act as a barrier. A well-known example of a vertical expansion project in the United States is the Cherry Island Landfill Expansion. The MSE berm created is approximately 60 feet high, two miles in length and added 19 years of disposal life to the landfill (Sevenson, 2021). Hullings et al., 2019 indicate that the ideal choice of soils for MSE walls are free-draining sands and gravels. The reinforcements can be materials such as metal strips, high-strength polyester geotextiles, or geogrids that add strength to the soil so that one or both slopes can be steeper than that soil’s natural angle of repose (Hullings, Piedmont-Fleischmann, & Shurie, 2019). Media’s, 2002 work shows that this technology allows MSE walls to be constructed at near-vertical slopes of 1:3 (H:V) to 1:6. The design of the reinforcing elements and the facing of the berm help determine its slope. As mentioned before, the internal reinforcement material can be steel, geotextile, or geogrid while the face can be reinforced with wire baskets, gabions, or concrete elements in order to control surface erosion (Hullings, Piedmont-Fleischmann, & Shurie, 2019).
The most common material used for internal reinforcement of MSE berms at landfills is geogrids- “a geosynthetic material consisting of connected parallel sets of tensile ribs with apertures of sufficient size to allow strike-through of surrounding soil, stone, or other geotechnical material” (Media, 2002). The geogrid is meant to hold the wedge of soil that would otherwise move outward in place (Media,2002). Tensile stresses, that are developed as a result of the steepened face of the MSE wall, are transferred to the geogrid in the form of a pullout force by means of the “frictional resistance between the geogrid and the soil on both the upper and lower surfaces of the longitudinal and transverse ribs” and “by means of bearing resistance against the front surface of the transverse ribs” (Media, 2002). The amount of stress transferred to the geogrid is dependent on the geogrid’s geometric structure and the surrounding soil properties (Media, 2002). Geogrids can be uniaxial or biaxial as shown in Fig. 1 and 2, depending on the orientation of the load-carrying ribs. MSE berms often have several geogrid layers of varying length, tensile strength, and vertical spacing to account for the many types of failure planes that can develop (Media, 2002).
Wire baskets are most commonly used as facing reinforcement at landfills because they are cheaper and more flexible than masonry alternatives, allowing for more accommodation of minor settlement and horizontal displacement (Hullings, Piedmont-Fleischmann, & Shurie, 2019). Media, 2002 describes this method, which involves
"extending the biaxial geogrid layers beyond the edge of the wall and wrapping them over the layer of soil that is placed over the geogrid. Biaxial geogrids are used for wrapping the face of the MSE wall and as secondary reinforcement near the face since they are more capable of resisting shallow, multidirectional-type failures that might occur near the face. Welded wire forms are used to ensure that the layer is constructed at the proper setback from the previous layer and to aid in the placement of geosynthetics. Secondary biaxial geogrid is then rolled out parallel to the wall face, such that a portion of the geogrid overhangs the welded wire forms. Erosion control matting is placed over the biaxial geogrid for soil retention. The next lift of soil is placed and compacted over the biaxial geogrid and erosion control matting to the top of the welded wire forms, and the free end of the geogrid is wrapped over the top of the soil lift. The facing materials are structurally tied into the wall by the primary uniaxial geogrid layers that are extended to the wall face. The process is repeated for each subsequent lift of soil." (Media, 2002)
An example of this design method is shown in Fig. 3.
Piggyback landfills are not appropriate for all sites, but they can be quite a good alternative when compared in a cost-benefit analysis to other timelier methods. Law et al., 2008 determined that permits for vertical expansions of landfills have been found to be faster than the process for a new landfill site. The permit process is specific to site jurisdictions and commonly includes an environmental impact report. The report often includes technical considerations such as “global slope stability under both static and seismic loading (if present) conditions, landfill base settlement, integrity of the bottom liner system and leachate piping, and stormwater management” in addition to matters such as aesthetics in which it must be evaluated whether or not the proposed project will “create a new source of substantial light or glare that is substantially greater than typical urban sources and could cause sustained annoyance or hazard for nearby sensitive receptors; or substantially interfere with an important scenic resource or substantially degrade the view of an existing scenic resource” (Law, Goudreau, Fawole, & Trivedi, 2008; L and D Landfill Vertical Expansion , 2018). According to Hullings et al., 2019, using an MSE berm as a component of a landfill expansion has the following benefits:
Creates additional disposal capacity over top of an existing landfill footprint
Uses existing facility operations infrastructure
Incorporates existing environmental management controls
Lessens irretrievable commitment of resources compared with the land commitment that would be associated with a lateral expansion
Decreases footprint compared to traditional earth embankment
Can be less costly than a traditional earth embankment because of the volume reduction
Avoids a potentially long and costly facility siting process
Increases capacity without increasing closure cost and with negligible impact on post-closure costs
Taking full advantage of the landfill’s existing infrastructure plays a crucial role in cost savings. When comparing the first cell of a hypothetical full buildout of a new landfill in the U.S., the cost may be close to $10 to $20 per cubic yard (CY), after applying all of the capital cost associated with permitting and construction of the initial landfill infrastructure versus $5 to $10 per CY of airspace for a lateral expansion of an existing landfill without an MSE berm (Hullings, Piedmont-Fleischmann, & Shurie, 2019). In terms of yielding the lowest cost per CY of created airspace, the optimum MSE berm height is in the range of 50 feet high – because afterwards stronger, more expensive reinforcement is needed (Hullings, Piedmont-Fleischmann, & Shurie, 2019). Figure 4 illustrates the relationship between berm height and airspace volume gained.
Landfills that are considering vertical expansion should take a few geotechnical aspects into account before the expansion begins. The original design may need to be modified in regard to global slope stability, settlement, and other possible failure modes. The weight of added material, as well as change in geometry of the landfill, create extra stresses that could cause failure under initial design conditions. These systems also need to be designed to remain stable over a long time period, and include structures that are affected by additional weight.
There are a few different types of stability considerations that must be analyzed in order to ensure that vertical expansion is viable. The first, and most straightforward, is the addition of weight that can increase driving forces of slip for the existing landfill. In order to explore this scenario, a basic slope stability analysis was performed. The calculation is based on the Suzhou Qizishan landfill that is discussed in Rong et al., 2011. Engineering properties and the description of the geometry of the landfill that are described in that report were used in this calculation.
The calculation of factor of safety against slope stability made use of an infinite slope analysis. Since only a general understanding of the effect of vertical expansion was the aim of this paper, the calculation used a few assumptions to simplify the analysis.
Key assumptions used were:
In order to gain the most accurate and representative results, a few cases were considered. The first was the stability of the landfill before vertical expansion. This serves as the baseline to compare other values with. The second case is for the full capacity of the vertical expansion. This will give a measure of the stability once the landfill is filled with waste to the capacity of the new expansion. Finally, the third case is for the stability of the waste in the vertically expanded area only. Each case was analyzed using a weighted average for the engineering properties of the waste, as well as the critical values. The following equations were used in the analysis.
For the cases 1 and 3:
For case 2:
The equations are essentially the same, but the second form was needed for case 2 due to the difference in slope of the landfill after expansion. The weight, W, couldn’t be normalized by the width, b, of the slope before the change in weight due to the vertical expansion was calculated. The results of the analysis are given in Table 1 below, as well as in Fig. 4.
Figure 4: Results for the factor of safety of the analysis before and after vertical expansion, using the average and critical values of each property.
Based on these results, the factor of safety will decrease with the increase in weight above the original landfill. The decrease is not extreme, but it is enough to create the potential for slope failures under the given conditions.
The previous example considers only the failure of the landfill as if the waste behaved like a soil. This is acceptable for the purpose of gaining basic understanding, but for practical use, more conditions exist. The engineer must also consider the “soil-geosynthetic or geosynthetic-geosynthetic interface within the liner system and cover system”
There are several mechanics that contribute to the settlement in a landfill. These include decomposition, physical and chemical deterioration, raveling, and mechanical compression due to added weight. It is the last mechanism that is the most relevant for vertical expansion of existing landfills. The other ways that settlement is triggered should have been accounted for in the original design, but will still be present in the original site and expanded area.
In the case of vertical expansion, “municipal solid waste usually settles a considerable amount during the filling operation and by about 10 to 30% of their initial height”
Because there are a few forms of settlement that can occur in a landfill, preventing it from occurring is not a reasonable request. However, there are techniques that can be used in order to limit the settlement of the MSW. Methods that are used in order to minimize, or accelerate, consolidation in clay layers does not seem to be applicable to the layers of waste. A time and cost-efficient method for precompression in order to limit the damages caused by settlement cannot be practiced for an existing landfill site in which the load of the vertical expansion will be slowly added. Instead, there are reinforcement methods that can be used to mitigate settlement. One such method is the installation of “a geosynthetic reinforcement on top of the old waste in order to retain the loads made by the new waste”
Settlement of the material in the landfill can also help in the process of maximizing the airspace. As the waste material consolidates, the top elevation of the site will drop. Slowly, the amount of waste that can be placed in the site is increased. Although this is true, the negative effects of settlement outweigh the added benefit of greater capacity. An engineer could go through the appropriate steps in order to safely maximize airspace by allowing a controlled amount of settlement. This would require a thorough investigation of the stability, settlement, and material properties of the waste at the site in question. In the interest of safety and time efficiency, this form of analysis is not recommended.
Landfills also require long design lives, and need to be engineered under long term conditions that last many years. Conditions of the facility during construction of the vertical expansion are critical, but the landfill must also be designed for “a closure period lasting potentially hundreds of years”
Although it was not covered in the paper, reanalysis of the structures that are included in the original landfill should be carried out. Retaining structures placed to hold back the waste, or nearby soil, will have an excess load applied to them due to the weight of the vertical expansion. The effect on other structures, like gas or water collection systems, and the containment system should be assessed as well.
This paper concludes that vertical expansions to landfills can be a sound waste management solution for a site, depending on its conditions and history of proper maintenance. Vertical expansions involve building new landfill cells on top of existing ones in order to maximize use of the area already allocated for the landfill. The implementation of said expansions requires careful consideration of global slope stability, settlement, and other possible failure modes and the subsequent construction of new reinforcements that takes these considerations into account. Through an analysis of the Suzhou Qizishan landfill that is discussed in Rong et al., 2011 it was determined that the increase in weight associated with a vertical expansion can create the potential for slope failures under certain conditions. Analyses of this sort highlight the importance of designing landfill expansions carefully to prevent failure. Several other considerations are necessary and equally important to maintain the structural integrity of the system over the design life of the landfill.
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